The run-up to any budget, or indeed the lesser pre-budget report, is a game of cat and mouse in which the government tries to massage public expectations, and keep the opposition guessing.

In terms of managing expectations, the government may be struggling, as the planned fiscal stimulus rises from an initial £15bn to estimates of £30bn. But in terms of the chess game with the opposition, the government is claiming checkmate.

On fiscal stimulus, the Treasury hit trouble when the Financial Times reported that it would take 1% of GDP to have any real impact on the coming recession.

The 2008 budget had suggested that spending would be reined in from 2.7% of GDP in 2008-09, and to 2.2% in 2010-11, the equivalent of £7bn worth of higher taxes and spending cuts. So to create a stimulus, the government must scrub out that £7bn contraction.

The government is already committed to extra spending because it needs to continue to help the low-paid. The one-off compensation for the abolition of the 10p tax ratecost £2.7bn this year, and would cost the same again if kept in place.

The continued delay in increased fuel duty would cost another £600m, as would continuing the stamp duty holiday. So just to keep existing compensation in place, the cost would be £4.2bn. Taken together, that means the government is committed to a minimum extra spending package of over £11bn. The European commission is expected next Wednesday to advocate a 1% stimulus. But during the G20 summit in Washington, Dominique Strauss-Kahn, the IMF managing director, made the case for a fiscal stimulus of 2% of GDP.

Downing Street insisted as soon as it could that a £30bn UK stimulus was way beyond what Britain was looking at. But by citing the IMF as justification for what the government is about to do, Downing Street has found itself associated with that figure.

There have been reports of tension between the Treasury and Number 10 over the size of the multibillion-pound boost to the economy, with Brown eager to catch the eye and Alistair Darling looking at the prospect of a mountain of debt. In practice, the size of the stimulus is likely to be a good deal smaller, partly because Darling has been looking at the coming borrowing figures, and is contemplating £64bn for this year and possibly £90bn for next year.

As a result the government has also been trying to manage down expectations by highlighting the £5bn or so that will be saved through higher than expected efficiency savings as a result of the Gershon review of Whitehall spending.

That puts the fiscal stimulus closer to £20bn - at most £7bn of which could be extra spending, including help for small businesses and extra spending on housing, schools and hospitals, and the remainder tax cuts, or higher credits.

But Labour is not just interested in securing a stimulus. Brown wants to use the budget to muddy any blue water that David Cameron is seeking to create.

Many ministers are staggered that the Conservatives are backing a lower rate of spending in 2010-11 than set out in Labour spending plans. They see it as a tactical error by the Tories which gives Darling a green light to announce a similar slower rate of increase in spending for 2010-11. That would help the chancellor to show how he will get the public finances back in order, and deprive the Conservatives of anything unique to say.

Similarly, the higher than expected Gershon review savings of around £5bn are also designed to undermine the impact of any efficiency savings that the Tories planned to announce in the next few weeks.

This frees Brown to say he and the rest of the world, including the EU, are willing to take a coordinated stimulus that the Tories are opposing on the grounds that the level of borrowing makes further spending unjustifiable. The Conservatives are left with the gamble that the public will buy their story that taxes will have to rise after the election, and a stimulus now will simply not work, and only worsen the eventual reckoning after the election.

Drawing on European commission work, the Conservatives claim the structural deficit is about 3% of national income, or £45bn, requiring big tax rises or spending cuts before the economy stabilises.

The Tories are hoping big ideas on getting credit moving to small businesses will immunise them against charges that fiscal prudence means they are willing to let the recession run its course by coming up with big ideas on how to get credit moving to small business.

But for the moment, so long as Labour can keep expectations under control on the size of the stimulus, it seems the Conservatives are trailing in the wake of the global chancellor, aka Gordon Brown.